Field
This disclosure relates to treatment of wastewaters containing organic matter and nitrogen by using a new category of membrane bioreactor. In particular, the disclosure relates to a high flux, non-fouling membrane module. The disclosure also relates to a process whereby sulfate reduction is used in an anaerobic zone to reduce excess sludge production.
Background
A Membrane Bioreactor (MBR), depicted in FIG. 1, is a type of biological wastewater treatment process. It operates in the mode of activated sludge, i.e., using a suspended growth of biological floc, also known as sludge, composed of bacteria and protozoa processes to oxidize organic carbon in the wastewater. Nevertheless, instead of using a sedimentation process to separate the sludge from the mixed liquid, the MBR process makes use of microfiltration membranes with typical pore size of 0.1 μm to 1 μm to separate the sludge from the mixed liquid. The permeate flux across the membrane is limited to below 1 m3 per m2 of membrane per day.
To ensure proper functioning of the membrane, special operations have to be arranged to prevent biofouling of the membrane. These include keeping the air-to-water ratio above 25 to provide sufficient self-cleaning of the membranes; frequent backwashing of the membrane once every 30 minutes; and periodic chemical or off-tank cleaning of the membrane every one or two months. All these special operations would result in the increase in energy consumption and operation cost, as well as reduction of effective operational duration. Moreover, to protect the membrane, the influent wastewater has to pass through primary sedimentation and fine-screening of 1 mm before the entering the aeration tank.
Solids retention time (SRT) is the average time the sludge stays in the system, and can be defined by:
      total    ⁢                  ⁢    amount    ⁢                  ⁢    of    ⁢                  ⁢    sludge        (                  sludge        ⁢                                  ⁢        wastage        ⁢                                  ⁢        rate            ,              including        ⁢                                  ⁢        solids        ⁢                                  ⁢        in        ⁢                                  ⁢        the        ⁢                                  ⁢        effluent              )  
Hydraulic retention time (HRT), sometimes called “hydraulic residence time”, is the average time the water stays in the system, and can be defined by:
      total    ⁢                  ⁢    volume        influent    ⁢                  ⁢    flow    ⁢                  ⁢    rate  
The MBR process normally operates at a hydraulic retention time (HRT) of over 6 hrs and has a soluble Chemical Oxygen Demand (COD) removal efficiency similar to a conventional activated sludge process.
Although a number of sludge production minimizing processes have been developed recently, these processes lead to either high costs or the need for more space. The best option for sludge minimization is using a process which inherently has low sludge production. For COD conversion, low sludge production can be achieved by using hydrolysis and methanogenic fermentation or using sulfate as the electron acceptor instead of oxygen or nitrate.
Oxic-Settling-Anaerobic (OSA) is a wastewater treatment technique employing an anaerobic zone or tank, and is capable of efficiently reducing sludge production and improving the stability of process operation by adding an anaerobic sludge tank in the sludge return line. In an OSA process, depicted in FIG. 2, two biological reactors and a settling tank are needed, which requires a larger real estate footprint. The hydraulic retention time (HRT) for OSA process is usually in excess of 4 hours to reduce the production of excess sludge and remove organic matters.
The Membrane Enhanced Primary Treatment (MEPT) is a wastewater treatment process that resembles a membrane bioreactor (MBR) without primary sedimentation, hence its performance effectively combines a primary sedimentation and MBR process.